Transport is responsible for around a seventh of greenhouse gas emissions globally. Of these emissions almost two thirds are the result of passenger travel while the rest is due to freight.

So passenger travel is a big deal for climate.

In the chart above, which comes from our new eBook Emit This, we compare carbon intensity of different types of passenger transport on a per passenger kilometer basis. Using it we can explain some elements important to the development of a sustainable transport system.

1) Fuel Economy

Our chart today compares the carbon intensity of different transport modes, per passenger kilometer. The better fuel economy gets the lower emissions go. If you just look at the cars you’ll see the large car (15 MPG) has emissions almost three times that of the hybrid car (45 MPG).

By improving fuel economy we can get the same mileage while generating fewer emissions. Something that is achieved by making engines more efficient, vehicles lighter and bodies more aerodynamic. But even then combustion engines remain relatively inefficient and produce emissions at the tailpipe, so improving them is really just a stop-gap en-route to sustainable transport.

2) Occupancy

The cheapest and simplest way to lower the carbon intensity of a passenger kilometer is to stick more people in the vehicle. In each of the figures above car occupancy is assumed to be an average of 1.6 passengers (including the driver). But most cars are designed for 5 people.

If you take a look at the bus examples the importance of occupancy becomes even more stark. The local bus example has emissions seven times higher than the school bus. While there routes may vary a little they are both diesel buses. The main difference is that the school bus has very high occupancy.

With notable exception of flying public transport tends to have quite low carbon emissions, due largely to having relatively high occupancy.

3) Electrification

In the absence of breakthroughs in second generation biofuels electrification is the most important pathway to low carbon transport.

Electric cars using low carbon power have footprints less than half that of the best hybrid, even after you account for their larger manufacturing footprint. Right down the bottom of our chart is the high-speed EuroStar rail which used low carbon French electricity. Though not on our chart the lowest carbon transport on earth is probably electrified public transport in a place like Norway where electricity generation is almost carbon free.

While there is a natural tendency to obsess about the electrification of cars, there are lots of interesting innovations occurring in the electrification of rail, motorbikes, scooters and bikes.

4) Pedal power

They may be a bit low tech for some, but when it comes to carbon emissions bicycles are pretty cutting edge. Even when you account for the foodprint of excess energy used when cycling, the humble bike is incredibly low carbon.

Bikes have obvious limitations around speed and distance, but for short trips in places with good infrastructure they are hard to beat in terms of carbon. They also have a great synergy with public transport systems like intercity rail.

5) Urbanization

Each of the first four elements we have described above refers to improving the carbon intensity of transport. But emissions are a function of both how we travel and how far we travel. One thing that tackles both of these issues is the trend towards urbanization.

People who live in cities have lower transport emissions. Fuel economy may be lower in city traffic but that is more than made up for by the fact that city dwellers drive far less. Electrification of public transport is more economic and practical in cities. Occupancy on public transport systems is much higher. And access to infrastructure for both cycling and walking is often better.

In 1950 less than 30% of the world’s population lived in cites, by 2010 that figure was over 50%, and by 2030 it is expected to surpass 60%. This natural trend to urbanization is a huge opportunity to for lowering both distance travelled per person and the carbon intensity of that travel.

Those are our five elements of sustainable transport: fuel economy, occupancy, electrification, pedal power and urbanization.

Check out our free new eBook Emit This for more ideas on getting more life out of less carbon.

4 Comments

Michael P Totten 5 years ago

Hello Lindsay, I am curious where you derived your calculation of 17 g per passenger kilometer for bicycling. By my calculation that amounts to 320 miles per gallon, whereas for decades I have used an estimate that the extra kilocalories of food required for bicycling amounts to around 2000 miles per gallon. Can you explain your calculation? thanks.

Lindsay Wilson 5 years ago

Its not my calculation so much. More a central figure from a couple of studies. There are some things that move it about. For a high carbon diet (meat) it goes up, low carbon down. The lighter and fitter the cyclists the lower it is. One problem in these calcs is that some studies forget to take out the baseline energy use, resulting in higher figures.

Macabre 5 years ago

Fascinating. What are the equivalent figures for walking I wonder? Probably beats cycling.

On the issue of urbanisation I agree from personal experience (having moved from Melbourne to just outside Healesville 12 months ago) that country living brings much higher transport kms and emissions. School runs are a killer. I think the main issue is that the car brings the convenience of city living, so we country folk shut our eyes to the environmental damage we are causing.

Lindsay Wilson 5 years ago

I’ve seen a few takes on walking. They are generally about double that of cycling for an equivalent diet. Actually an electric bike running on low carbon electricity can go below cycling (dependent on the cyclists diet).